Power plants, such as some furnaces, internal and external combustion engines, or other hydrocarbon burning devices may emit pollutants such as carbon monoxides, nitrogen oxides (NOx, including NO and NO2), particulate matters, and sulfur oxides, to name a few. During the combustion process of these engines or fuel-burning plants, fuel ignites in the presence of oxygen. If excess oxygen is present, the local mixture is lean, and the combustion usually results in the emission of increased NOx. If excess fuel is present, the local mixture is rich, and the combustion will result in the emission of unburned hydrocarbon, soot, soluble organic fraction (SOF), and sulfates, generally termed particulate matter. Consequently, most engines emit a mixture of NOx and particulate matter.
Emission of many of these pollutants is regulated by governmental agencies, which mandate that engines—along with other fuel-burning plants—not exceed certain maximum limitations. In meeting these regulations, engine manufacturers are challenged with designing engines that both meet stringent exhaust regulations as well as provide fuel-efficient power conversion. Additionally, engine manufacturers may have to design engines that meet all of the regulations for the several different types of exhaust pollutants.
One approach to reducing NOx, for instance, includes using ammonia as a reductant or reducing agent for use with a Selective Catalytic Reduction (SCR) catalyst. This process reduces the NOx to gaseous nitrogen and water. However, some ammonia based systems may produce excess ammonia (for example, more ammonia than the system is capable of reacting and/or storing).
One proposed solution to dealing with excess ammonia is described in U.S. Pat. Pub. No. 2007/0289288 to Dawson et al. (“Dawson”). Dawson purports to disclose a “system and method for venting an on-board vehicle emissions treatment substance storage and distribution system that . . . coupl[es] the storage tank to the exhaust system upstream of an ammonia storage element to reduce and direct any escaping ammonia toward the rear of the vehicle away from a refueling location.” However, the capacity of the ammonia storage element may be limited.